The present invention relates to an avalanche photodiode that realizes high sensitivity, low noise, broad band, and low dark current at the same time.
The so-called superlattice avalanche photodiode (to be referred to hereinafter as an APD) that uses a superlattice for the multiplication layer has been used in recent years as an APD for optical communications in a wavelength of 1.3 xcexcm or 1.55 xcexcm. In general, multiplication noise of the APD decreases as a ratio between ionization rates of electrons and holes (xcex1 and xcex2), which is inherent to a semiconductor used for the multiplication layer, departs from value xe2x80x9c1xe2x80x9d. A superlattice structure is used for the multiplication layer in order to increase the ratio of the ionization rates xcex1/xcex2 or xcex2/xcex1. It has been known that, an In0.52Al0.48As/In0.8 Ga0.2 As0.6 P0.4 layer, which is lattice-matched with an InP substrate, in particular, has a large xcex1 value, and is therefore effective for achieving low noise, since it has nearly no discontinuity in the valence band, whereas discontinuity in the conduction band is large in the interface.
In addition, there has been proposed a structure, in which a light-absorbing layer is separated from a superlattice multiplication layer and that the light-absorbing layer is comprised of p-type, so as to differentiate an electric field intensity between the superlattice multiplication layer and the light-absorbing layer in order to control an avalanche breakdown within the light-absorbing layer, and to limit an area wherein avalanche amplification takes place only in the superlattice region. As an example of such a structure that satisfies all of the foregoing conditions, Japanese Patent Laid-open Publication, No. H02-298082, discloses a structure wherein a thin sheet-doping layer having a high concentration of p-type impurities is placed between a pxe2x88x92-InGaAs light-absorbing layer and a superlattice multiplication layer.
Further, as described in Japanese Patent Laid-open Publication, No. H02-282847, if a tunnel current is generated in the sheet-doping layer, the tunnel current can be controlled by using an layer having a larger band gap such as In0.52 Al0.48As layer, an InP layer, or an In0.8Ga0.2AS0.6P0.4 layer.
FIGS. 5A and 5B show an example of structure of a superlattice APD element of the prior art. FIG. 5A illustrates electric field intensity distribution when a reverse bias voltage is applied to this element, and FIG. 5B shows a cross sectional view of the element. In FIG. 5B, the superlattice APD element of the prior art comprises:
(a) an n+-InP substrate 501;
(b) an n+-InP buffer layer 502;
(c) a non-doped In0.52Al0.48As/In0.8Ga0.2As0.6P0.4 superlattice multiplication layer 503;
(d) a p-type InP layer (sheet-doping layer) 504 having an impurity concentration of 8xc3x971017 cmxe2x88x923 and a thickness of 160  xc3x85;
(e) a pxe2x88x92-type In0.47Ga0.53As light-absorbing layer 505 having impurity concentration of 2xc3x971015 cmxe2x88x923 and a thickness of 1 xcexcm;
(f) a p+-In0.47Ga0.53As layer 506 having an impurity concentration of 2xc3x971017 cmxe2x88x923 and a thickness of 500 xc3x85;
(g) a p-type InP window layer 507 having an impurity concentration of 1xc3x971018 cmxe2x88x923 and a thickness of 1000 xc3x85;
(h) a p+-In0.47Ga0.53As contact layer 508 having an impurity concentration of 1xc3x971018 cmxe2x88x923 and a thickness of 1000 xc3x85;
(i) an AuZnNi electrode and reflector (P-electrode) 509; and
(j) an AuGeNi electrode (N-electrode) 510.
In the foregoing structure, light incident from one side of the n+-InP substrate 501 is absorbed in the In0.47Ga0.53As light-absorbing layer 505, and pairs of electrons and holes are generated. The electrons travel toward the superlattice multiplication layer 503 responsive to the bias voltage applied between the AuZnNi electrode 509 and the AuGeNi electrode 510, and are injected into the layer. Because the AuZnNi electrode 509 also serves as a light-reflecting layer, the light incident from the side of n+-InP substrate 501 and not absorbed in the light-absorbing layer 505 is reflected by the AuZnNi electrode 509, and is then absorbed almost entirely when it passes again through the light-absorbing layer 505. Since the incident light is effectively used in the described manner, a quantum efficiency does not decrease even if a layer thickness of the light-absorbing layer 505 is a half of 2 xcexcm, which is a reciprocal number of its absorption coefficient. Moreover, a response of the APD increases, because the layer thickness of the light-absorbing layer 505 is reduced.
Furthermore, since the superlattice multiplication layer 503 has a sufficiently large ionization rate of electrons as compared with an ionization rate of holes, it realizes multiplication of the electrons injected into the superlattice multiplication layer 503, by means of a veritable electron injection, without increasing multiplication noises.
However, the above-described structure has problems. Described first is a technical problem inherent in the superlattice APD of the above structure.
As previously described, the electrons generated in the light-absorbing layer are injected into the superlattice multiplication layer by the applied electric field, and they are ionized as they receive energy corresponding to the discontinuity in the conduction band of the superlattice layer. The thinner the layer thickness of the superlattice multiplication layer, the shorter an avalanche progression time becomes, and hence the response increases, because the electrons are moving through each semiconductor layer during this period. However, a reduction in layer thickness of the multiplication layer lowers the multiplication factor, since it decreases probability of the ionization. Moreover, it reduces the xcex1/xcex2 ratio between the ionization rate xcex1 of electrons and the ionization rate xcex2 of holes, because it tends to retard ionization of the electrons, thereby increasing the excess noises at the same time.
Another improved APD is disclosed in U.S. Pat. No. 5,471,068, where a strain is applied to at least one of well layer and superlattice avalanche multiplier layer to decrease the energy difference between lower end of conduction band of the well layer and the barrier layer, or to increase the energy difference between the upper end of the valence band of them. But, APD using the above strained layer could not sufficiently decrease the energy difference between lower end of conduction band of the well layer and the barrier layer to zero level.
The present invention aims to address the above shortcomings that are inherent to the superlattice APD of the prior art, so as to decrease the dark current, greatly improve high frequency characteristics, and also reduce the operating voltage at the same time by reducing the energy difference between lower end of conduction band of the well layer and the barrier layer to nearly zero.
The present invention introduces a distortion-compensated superlattice into a superlattice multiplication layer in the superlattice APD, so as to increase discontinuity xcex94Ec in a conduction band while maintaining discontinuity in a valence band to nearly zero. In other words, in the APD of the present invention, discontinuity in a valence band is made nearly zero by introducing InGaAsP layer as a well layer, while strain is introduced to the superlattice multiplication layer to increase xcex94Ec. This structure is effective to increase an electron multiplication factor and also effective to decrease pileup of holes. The structure of the present invention increases an ionization rate xcex1 of electrons, because it increases the xcex94Ec, and hence a ratio xcex1/xcex2 as well. Accordingly, the multiplication factor increases, and excess noise is reduced. Because an effective band gap Eg,eff of the superlattice layer increases, on the other hand, a dark current is reduced. In addition, the present invention provides a reflective multi-layered structure for a light-reflecting layer of the superlattice APD in order to improve utilization efficiency of incident light. This structure of the present invention provides for the possibility of reducing a layer thickness of the superlattice multiplication layer without decreasing a multiplication factor, thereby realizing the superlattice APD of low operating voltage while increasing a response thereof.